Aircraft engines generate sound and heat in their operation. Excessive sound is undesirable largely because of disturbance to surrounding communities. Heat is undesirable particularly in both military and civil aircraft, which may be tracked by ground-based missiles that seek heat in the form of infrared radiation. The design of the aircraft profoundly impacts the sound and heat that are observed from the ground.
Conventional subsonic civil aircraft designs commonly feature engine placement underneath the airplane's wings. Conventional supersonic military aircraft designs commonly feature engine placement in the aft-most portion of the airplane with the nozzles extending aft of the wing and control surfaces. The sound pressure level produced by the engines, herein generally referred to as noise, and particularly jet noise or the “roar” heard at takeoff, travels largely unabated to communities. For under-wing engine installations this noise is amplified by the under-surface of the wing because the portion of the sound produced by the engines that would otherwise radiate upward is reflected downward off of the under-surface. The jet plume interacts with the wing trailing edge. Both the under-surface reflection and the jet plume interaction with the wing trailing edge add to the overall noise heard below. Even when engines are located higher than wings, aircraft generally offer little in the way of impeding the downward travel of sound due to the absence of a surface the covers a substantial extent of the downward sound propagation path. Technological improvements in engines have resulted in a gradual reduction of engine noise over time, but further reductions based on similar improvements will likely be minimal.
Heat similarly radiates from aircraft engines. Both military and civil aircraft must now contend with hostile environments that may include man-portable air defense systems (MANPADS). Some conventional airplane designs having the engines mounted beneath the wings, or the exhaust at the rear end of the aircraft, radiate heat unimpeded to the ground. This issue is exacerbated by the conventional under-wing mount, because radiant heat energy also reflects off the pylon and underside of the wing, much like noise energy. Countering the looming threat of heat-seeker missiles to transport aircraft operating in hostile airspace can involve two approaches. First, self-defense countermeasure systems may be added to the aircraft. Second, the observability of the major contributors to heat signature may be reduced. With respect to some countermeasures employments, the under-wing engine installation could interfere with the optimum countermeasure device placements. Furthermore, the jet plume can interact with the wing trailing edge in a way that splits or spreads the exhaust plume, potentially increasing vulnerability of aircraft outfitted with self-defense countermeasure systems. Currently available technologies that sufficiently reduce the observability of the heat signature in complement with self-defense systems generally have negative impact on performance, have limited safety improvement, and are expensive.
In the case of supersonic aircraft, the propulsion system also contributes to the sonic boom produced during supersonic flight. Reduction of sonic boom from typical levels is widely believed to be necessary for regulators to ever accept civil supersonic flight. The characteristic N-wave of a sonic boom is created both by shockwaves produced at the fore and aft regions of the aircraft. Strides have been made at reducing fore shocks. An appreciable reduction in sonic boom annoyance, however, cannot be realized without reduction of both fore and aft shocks, a portion of which is typically produced by the propulsion system.